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This means both banks have a lean condition. Most likely if you have a 4.0 SOHC motor your upper intake is sucking in air at the orings at the bottom of the intake. These are really bad about when they are cold in the morning the shrink and let air get by and after the engine warms up the orings soften and seal off.

The plastic piece is the upper intake manifold. The metal part is the lower intake manifold. Sometimes the gaskets go bad between the upper and lower intake manifolds. But usually the upper intake manifold is cracked and needs replaced.

Ha, you tried to fool me with the bit about a 3.5 L motor--I could find that one...here is the complete procedure from AutoZone.com:

Installation
To install:

Clean the gasket mounting surfaces. Be sure to inspect the manifold for warpage and/or cracks. If necessary, replace it.

Position the gaskets on the cylinder head with the port blocking plates to the rear and the This Side Up stamps facing upward. Then apply a 3 / 16 in. (5mm) bead of RTV sealant on the front and rear of the engine block at the block-to-manifold mating surface. Extend the bead 1 / 2 in. (13mm) up each cylinder head to seal and retain the gaskets.

Fig. Lower intake manifold tightening sequence-4.3L engines

Install the lower intake manifold. Tighten the bolts in sequence and in 3 steps, as follows:

Temporarily reattach the negative battery cable, then pressurize the fuel system (by cycling the ignition without starting the engine) and check for leaks.

Disconnect the negative battery cable.

Install or connect the following:

Heater hose to the lower intakeUpper radiator hose to the thermostat housingDistributor assembly and engage the wiringIgnition coilConnect vacuum hoses to the upper and lower intake manifold.New upper intake manifold gasket, making sure the green sealing lines are facing upwardUpper intake manifoldManifold retainers. Tighten them to 88 inch lbs. (10 Nm) using two passes.Purge solenoid and bracketBrake booster vacuum hose at the upper intake manifoldPCV hose to the rear of the upper intake manifoldVacuum hoses to both the front and rear of the manifold assemblyThrottle bodyThrottle linkage to the upper intakeWiring to the upper intake components including the TP sensor, IAC motor, MAP sensor and the fuel meterEngine coverAir intake assembly

Carefully place the upper intake manifold onto the lower intake manifold. Ensure that the alignment pins in the upper intake manifold align with the holes in the lower intake manifold.
NOTE: Apply thread lock compound, to the bolt threads before assembly.

NOTE
The 3.8L (VIN K) non-supercharged engine uses a two-piece intake
manifold consisting of an upper air plenum which mounts the throttle
body and a lower intake manifold assembly which houses the fuel
injectors. The 3.8L (VIN 1) supercharged engine uses a similar lower
intake manifold. The supercharger serves as the upper manifold.Lower Manifold

NOTE
Two bolts which fasten the lower intake manifold to the cylinder
head are accessible only after the upper intake manifold is removed.
These bolts are located in the right front and left rear corners of the
lower intake manifold. Remove the upper intake manifold to service the
lower intake.The 3.8L (VIN 1) supercharged engine uses a manifold
similar to the 3.8L (VIN K), but the supercharger serves as the upper
manifold.

Disconnect the negative battery cable.

On 3.8L (VIN 1) engines, remove the supercharger assembly. Please see the procedure in this section.

Remove the lower intake manifold bolts and carefully lift off the manifold.

To install:

Fig. Lower intake manifold bolt torque sequence-3.8L (VIN K) engine

Clean all parts well with degreaser, especially the gasket seal
surfaces on the upper and lower manifold pieces and between the lower
manifold and the cylinder heads..

Install new gaskets to the cylinder heads and new seals to the
engine block. Carefully lower the manifold into place. Apply
thread-locking compound to the bolt threads. Make sure to install the
two hidden bolts in the lower intake manifold. Hand start all bolts,
then torque evenly to 11 ft. lbs. (15 Nm). Follow the toque sequence,
starting in the center, then working outwards in a circle.

Install the remaining components in the reverse order of the removal process.

Upper Intake Manifold (Plenum)

Disconnect the negative battery cable.

On 3.8L (VIN 1) engines, remove the supercharger assembly. Please see the procedure in this section.

Tag for identification, then remove the right side spark plug wires.

Disconnect the ignition wires from the fuel rail.

Remove the fuel rail. Please see Section 5.

Remove the cable bracket from the intake manifold.

Remove the throttle body.

NOTE
Two bolts which fasten the lower intake manifold to the cylinder
head are accessible are accessible only after the upper intake manifold
is removed. These bolts are located in the right front and left rear
corners of the lower intake manifold.

Remove the upper intake manifold bolts and carefully lift the upper intake manifold from the engine.

Clean all parts well with degreaser, especially the gasket seal surfaces on the upper and lower manifold pieces.

Install new gaskets to the cylinder heads and new seals to the
engine block. Carefully lower the manifold into place. Make sure to
install the two hidden bolts in the lower intake manifold. Hand start
all bolts, then torque evenly to 89 inch lbs. (10 Nm). Follow the
torque sequence, starting in the center, then working outwards in a
circle.

Install the remaining components in the reverse order of the removal process.

if you have the 3.8 liter its more than likely bad intake manifold gaskets or a cracked manifold. Ford has had several TSBs on this issue. O2 sensors do not go bad as often as mechanics say they do. I have a 1999 that after 100 dollars at the parts store solved all lean conditions on the top end. The manifold has three parts to it. A lower aluminum that houses the butterfly blades for the variable intake runners( known vacuum leak were rods protrude at passenger side of lower bad bushings likely cause). Middle riser which uses oring seals which dry out and cause major vacuum leaks. upper is more or less the top cover which uses one large oring to seal. Hope this helps!

There are 2 common problems on this concern. 1st maybe a dirty MAF sensor. Remove the MAF sensor and clean it with a good electrical cleaner and air dry or light shop air,then reinstall and do a couple of wide open throttle burst to reset it.
2nd problem and we see alot of this is the orings between the upper and lower intake manifold leaks.You would have to remove the upper intake manifold and replace them.